I'm wonder how much WEM can leverage external social power (like Facebook, Twitter, etc) into WEM based web site.
What are capabilities of WEM in this perspective?
For example, WEM authoring user can post comments on Facebook page, or Twitter? (any standard function)
Also can WEM get any information stored in Facebook and display them on own website? (pull social-graph)
Any other special funtion around social integration?
Technically, Adobe WEM follows the same rules about Facebook/Twitter integration as a standard Java web application. Really that's all WEM is anyway...it is just built on top of a content repository and a platform for content manipulation (the WCM part). Whatever the Facebook/Twitter/whatever APIs allow you to do is possible in Adobe WEM. To implement, you would probably have to build Java components that manage whatever WEM-social network communication you are trying to accomplish, but that's just speaking in general terms.
However, if you are asking whether WEM has out-of-the-box, "enter username and password here, then voila!" Facebook/Twitter integration, then no it doesn't to my knowledge.